Gallium nitride crystal and method of making same

ABSTRACT

There is provided a GaN single crystal at least about 2.75 millimeters in diameter, with a dislocation density less than about 10 4  cm −1 , and having substantially no tilt boundaries. A method of forming a GaN single crystal is also disclosed. The method includes providing a nucleation center, a GaN source material, and a GaN solvent in a chamber. The chamber is pressurized. First and second temperature distributions are generated in the chamber such that the solvent is supersaturated in the nucleation region of the chamber. The first and second temperature distributions have different temperature gradients within the chamber.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of application Ser. No.11/376,575, filed on Mar. 15, 2006 and of U.S. Pat. No. 7,098,487 issuedon Aug. 29, 2006. This application claims priority to and benefit fromthe foregoing, the disclosures of which are incorporated herein byreference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH & DEVELOPMENT

The United States Government may have certain rights in this inventionpursuant to Cooperative Agreement No. 70NANB9H3020, awarded by theNational Institute of Standards and Technology, United States Departmentof Commerce.

BACKGROUND OF THE INVENTION

This invention is related generally to a high quality gallium nitridesingle crystal and a method of making the same.

FIELD OF THE INVENTION

Gallium nitride (GaN) based optoelectronic and electronic devices are oftremendous commercial importance. However, the quality and reliabilityof these devices is compromised by very high defect levels, particularlythreading dislocations in semiconductor layers of the devices. Thesedislocations can arise from lattice mismatch of GaN based semiconductorlayers to a non-GaN substrate such as sapphire or silicon carbide.Additional defects can arise from thermal expansion mismatch,impurities, and tilt boundaries, depending on the details of the growthmethod of the layers.

The presence of defects has a deleterious effect on epitaxially-grownlayers, compromising electronic device performance and requiringcomplex, tedious fabrication steps to reduce the concentration and/orimpact of the defects. While a substantial number of growth methods forgallium nitride crystals have been proposed, the methods to date stillmerit improvement.

U.S. Pat. Nos. 5,637,531 and 6,273,948 disclose methods for growinggallium nitride crystals at high pressure and high temperature, usingliquid gallium and gallium-based alloys as a solvent and a high pressureof nitrogen above the melt to maintain GaN as a thermodynamically-stablephase. The process is capable of growing electrically-conductive GaNcrystals with a dislocation density of about 10³-10⁵ cm⁻² or,alternatively, semi-insulating GaN crystals with a dislocation densityof about 10-10⁴ cm⁻², as described by Porowski, “Near defect-free GaNsubstrates” [MRS Internet J. Nitride Semicond. Research 4S1, G1.3(1999)].

The conductive crystals, however, have a high concentration of n-typedefects, on the order of 5×10¹⁹ cm⁻³. These defects are believed tocomprise. oxygen impurities and nitrogen vacancies. As a consequence,the crystals are relatively opaque, with an absorption coefficient ofabout 200 cm⁻¹ at wavelengths in the visible portion of the spectrum. Asa consequence, up to half the light emitted by a light emitting diode(LED) fabricated on such a crystal is absorbed by the substrate. Thisconstitutes a large disadvantage compared to conventionalheteroepitaxial LEDs fabricated on sapphire or transparent SiCsubstrates. Further, the high concentration of n-type defects innominally undoped crystals grown in molten Ga causes the latticeconstant to increase by about 0.01-0.02%, which generates strain inundoped epitaxial GaN layers deposited thereupon. Additionally, theundoped GaN substrates formed by this method have a rather limitedcarrier mobility, about 30-90 cm²/V-s, which may be limiting inhigh-power devices.

The transparency and dislocation density of GaN crystals grown in liquidGa may be improved by the addition of Mg or Be to the growth medium, butthe crystals so produced are semi-insulating, with a resistivity aboveabout 10⁵ Ω-cm. Such crystals are not appropriate for vertical devicesin which one electrical contact is made to the substrate itself.

The most mature technology for growth of pseudo-bulk or bulk GaN ishydride/halide vapor phase epitaxy, also known as HVPE. In themost-widely applied approach, HCl reacts with liquid Ga to formvapor-phase GaCl, which is transported to a substrate where it reactswith injected NH₃ to form GaN. Typically the deposition is performed ona non-GaN substrate such as sapphire, silicon, gallium arsenide, orLiGaO₂. The dislocation density in HVPE-grown films is initially quitehigh, on the order of 10¹⁰ cm⁻² as is typical for heteroepitaxy of GaN,but drops to a value of about 10⁷ cm⁻² after a thickness of 100-300 μmof GaN has been grown.

HVPE may be capable of reducing defect levels further in thicker films,but values below 10⁴ cm⁻² over an entire wafer seem unlikely. Inaddition, strain is present in HVPE wafers due to the thermal expansionmismatch between substrate and film. This strain produces bowing uponcool down of the substrate and film after growth, which remains evenafter removal of the original substrate.

For reasons that are not yet understood, in thick HVPE GaN neitherabsorption nor emission of light at room temperature occurs with athreshold at the band edge. In transmission spectroscopy, HVPE GaNabsorbs with a cutoff near 370 nm, significantly shifted from theexpected cutoff near 366 nm. Similarly, the photoluminescence peak atroom temperature occurs at 3.35 eV, at significantly lower energy thanexpected. This behavior will compromise the performance of lightemitting devices operating in the ultraviolet, as some of the light willbe absorbed by the substrate rather than being emitted. The shiftedphotoluminescence peak indicates the presence of defect states that maycompromise device performance.

Other widely-applied methods for growth of large area,low-dislocation-density GaN are variously referred to as epitaxiallateral overgrowth (ELO or ELOG), lateral epitaxial overgrowth (LEO),selective area growth (SAG), dislocation elimination by epitaxial growthwith inverse pyramidal pits (DEEP), or the like. In the case of allvariations of this method, heteroepitaxial GaN growth is initiated in aone- or two-dimensional array of locations on a substrate, where thelocations are separated by a mask, trenches, or the like. The individualGaN crystallites grow and then coalesce. Epitaxial growth is thencontinued on top of the coalesced GaN material to produce a thick filmor “ingot.” Typically, the thick GaN layer formed on the coalesced GaNmaterial is deposited by HVPE.

This process is capable of large reductions in the concentration ofdislocations, particularly in the regions above the mask. However, theresulting GaN substrate is not a true single crystal although a numberof authors do refer to ELO structures as single crystals. Eachindividual GaN crystallite constitutes a grain, and there is typically alow-angle grain boundary or a tilt boundary at the points where thegrains coalesce. The low-angle or tilt boundaries are manifested as anarray of edge dislocations, and generate lateral strain within the GaN.The magnitude of the crystallographic tilting depends on the details ofthe masking and growth conditions, but there is generally at least a lowlevel of tilting associated with grain coalescence. Much or most of thecrystallographic tilting forms directly during growth, rather thansimply, being a consequence of thermal expansion mismatch.

The tilt-grain-boundary structure and lateral strain persists throughoutan entire ingot and therefore into each substrate sliced from thisingot. In other words, no substrate sliced from such an ingot will be atrue single crystal, free of tilt boundaries and lateral strain. Inaddition, the GaN substrate is likely to suffer from the samedeficiencies in UV absorption and photoluminescence at room temperatureas “standard” HVPE GaN.

Other methods for crystal growth of GaN involve the use of supercriticalammonia as a solvent. Several groups have reported growth of very smallGaN crystals in supercritical ammonia, notably Kolis et al. in“Materials Chemistry and Bulk Crystal Growth of Group III Nitrides inSupercritical Ammonia”, Mater. Res. Soc. Symp. Proc. 495, 367 (1998)];“Crystal Growth of Gallium Nitride in Supercritical Ammonia”, J. Cryst.Growth 222, 431 (2001); “Synchrotron white beam topographycharacterization of physical vapor transport grown AlN and ammonothermalGaN”, J. Cryst. Growth 246, 271 (2002); and Dwilinski et al. in “AMMONOMethod of GaN and AlN Production”, Diamond Relat. Mater. 7, 1348 (1998);“AMMONO Method of BN, AlN, and GaN Synthesis and Crystal Growth”, MRSInternet J. Nitride Semiconductor Res. 3, article 25 (1997); “On GaNCrystallization by Ammonothermal Method” Acta Phys. Pol. A 90, 763(1996); and “GaN Synthesis by Ammonothermal Method” Acta Phys. PolonicaA 88, 833 (1995)]. However, only small crystals or mm-sized crystals ofrather poor quality have been reported to date. In addition, theseauthors do not disclose the use of temperature gradient profiles tooptimize growth on seeds.

French patent FR 2,796,657 to Demazeau et al. discloses a method for GaNgrowth in supercritical ammonia or hydrazine at pressures of 0.05-20kbar, temperatures of 100-600° C., and a temperature gradient of 10-100°C. The only apparatus taught by Demazeau to access these conditions is aTuttle-type cold-seal pressure vessel, which is well known in the artand is limited to a maximum pressure of 5-6 kbar. Standard pressurevessels are limited to a pressure of about 5-6 kbar when working withNH₃, as discussed by Jacobs and Schmidt in “High Pressure Ammonolysis inSolid-State Chemistry”, Curr. Topics Mater. Sci. 8, ed. by E Kaldis(North-Holland, 1982)], limiting the maximum temperature, reaction rate,and, in all likelihood, crystalline quality. Therefore, Demazeaudiscloses no method capable of reaching the higher pressure range, anddoes not demonstrate GaN crystals larger than 1 mm in size. In addition,Demazeau does not teach the use of temperature gradient profiles tooptimize growth on seeds.

U.S. Pat. No. 6,398,867 to D'Evelyn et al. discloses a method fortemperature gradient recrystallization of GaN in a supercritical fluidat a pressure greater than 5 kbar, a temperature greater than 550° C.,with a temperature gradient of 5-300° C.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, there isprovided a GaN single crystal at least about 2.75 millimeters indiameter, with a dislocation density less than about 10⁴ cm⁻¹, and beingsubstantially free of tilt boundaries.

In accordance with another aspect of the present invention, there isprovided a GaN single crystal at least about 2 millimeters in diameter,and having no tilt boundaries, wherein the single crystal has aphotoluminescence spectrum which peaks at a photon energy of betweenabout 3.38 and about 3.41 eV at a crystal temperature of 300° C.

In accordance with another aspect of the present invention, there isprovided a method of forming a GaN single crystal. The method comprises(a) providing a nucleation center in a first region of a chamber; (b)providing a GaN source material in a second region of the chamber; (c)providing a GaN solvent in the chamber; (d) pressurizing the chamber;(e) generating and holding a first temperature distribution such thatthe solvent is supersaturated in the first region of the chamber andsuch that there is a first temperature gradient between the nucleationcenter and the GaN source material such that GaN crystal grows on thenucleation center; and (f) generating a second temperature distributionin the chamber such that the solvent is supersaturated in the firstregion of the chamber and such that there is a second temperaturegradient between the nucleation center and the GaN source material suchthat GaN crystal grows on the nucleation center, wherein the secondtemperature gradient is larger in magnitude than the first temperaturegradient and the crystal growth rate is greater for the secondtemperature distribution than for the first temperature distribution.

In accordance with another aspect of the present invention, there isprovided a method of forming a GaN single crystal. The method comprises(a) providing a nucleation center in a first region of a chamber havinga first end; (b) providing a GaN source material in a second region ofthe chamber having a second end; (c) providing a GaN solvent in thechamber; (d) pressurizing the chamber to a pressure of between 5 and 80kbar; (e) generating and holding a first temperature distribution havingan average temperature between about 550° C. and about 1200° C. suchthat the solvent is supersaturated in the first region of the chamberand such that there is a first temperature gradient between the firstend and the second end such that GaN crystal grows on the nucleationcenter; and (f) generating a second temperature distribution in thechamber having an average temperature between about 550° C. and about1200° C. such that the solvent is supersaturated in the first region ofthe chamber and such that there is a second temperature gradient betweenthe first end and the second end such that GaN crystal grows on thenucleation center, wherein the second temperature gradient is larger inmagnitude than the first temperature gradient and the crystal growthrate is greater for the second temperature distribution than for thefirst temperature distribution.

In accordance with another aspect of the present invention, there isprovided a method of forming a GaN single crystal. The method comprises(a) providing a nucleation center in a first region of a chamber havinga first end; (b) providing a GaN source material in a second region ofthe chamber having a second end; (c) providing a GaN solvent in thechamber; (d) pressurizing the chamber; (e) generating and holding afirst temperature distribution such that there is a first temperaturegradient between the first end and the second end; and (f) generating asecond temperature distribution in the chamber such that the solvent issupersaturated in the first region of the chamber and such that there isa second temperature gradient between the first end and the second endsuch that GaN crystal grows on the nucleation center, wherein the firsttemperature gradient is zero or opposite in sign from the secondtemperature gradient.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional representation of a capsule usedfor making a GaN single crystal according to a preferred embodiment ofthe invention.

FIG. 2 is a schematic cross-sectional representation of a pressurevessel used for making a GaN single crystal according to a preferredembodiment of the invention.

FIG. 3 is a series of photoluminescence spectra of a GaN crystalaccording to a preferred embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to presently preferred embodimentsof the present invention. Wherever possible, the same reference numberswill be used throughout the drawings to refer to the same or like parts.

Crystalline composition defects refers to one or more of point defects,such as vacancies, interstitials, and impurities; one-dimensional lineardefects, such as dislocations (edge, screw, mixed); two-dimensionalplanar defects, such as tilt boundaries and grain boundaries; andthree-dimensional extended defects, such as voids, pores, pits, andcracks. Defect may refer to one or more of the foregoing unless contextor language indicates that the subject is a particular subset of defect.Free of tilt boundaries or substantially free of tilt boundaries meansthat the crystalline composition may have tilt boundaries at aninsubstantial level, or with a tilt angle such that the tilt boundariesmay not be readily detectable by TEM or X-ray diffraction; or, thecrystalline composition may include tilt boundaries that are widelyseparated from one another, e.g., by at least 2.75 millimeters or by agreater, and specified, distance. Thus, “free” or “substantially free”may be used in combination with a term, and may include an insubstantialnumber or trace amounts while still being considered free of themodified term, and “free” or “substantially free” may include furtherthe complete absence of the modified term.

In a GaN crystal, there are three dimensions as represented by x,y,wcharacters, with w as the thickness, and x and y the dimensions of thecrystal plane perpendicular to w. For a round or circular crystal,x=y=the diameter of the crystal.

As used herein, a boule may be used interchangeably with an ingot,referring to a crystal having a volume in excess of 0.2 cm³ with aminimum thickness (z dimension) of 0.2 cm.

The present inventors have found that GaN nucleates very readily insupercritical ammonia and other supercritical GaN solvents so thatrecrystallization produces many small crystals rather than one largecrystal. While small high quality GaN crystals could be grown by themethods known in the prior art, no high quality crystal larger than 2.75mm (in the x or y dimension) has been grown by these methods. Thepresent inventors have found that a method using an improved temperatureprofile including appropriate temperature gradients, together with animproved method for mounting seed crystals, is capable of overcomingthese limitations.

According to embodiments of the present invention, a true single crystalmay be synthesized, and grown from a single nucleus to a size of atleast 2.75 mm in at least one of x and y dimensions. If the crystal iscircular or somewhat circular in shape, the single crystal is at least2.75 mm in diameter. The single crystal may be n-type, electricallyconductive, optically transparent, free of lateral strain and tiltboundaries, and with a dislocation density less than about 10⁴ cm⁻².Preferably, the dislocation density is less than about 10³ cm⁻². Evenmore preferably, the dislocation density is less than about 100 cm⁻².

This large high quality gallium nitride single crystal may be grown bytemperature gradient recrystallization at high pressure and hightemperature in a supercritical fluid solvent. The crystal is a truesingle crystal, i.e., it is substantially free of tilt boundaries.

These gallium nitride crystals may be grown by temperature-gradientrecrystallization in a supercritical fluid, including but not limited toammonia, hydrazine, methylamine, ethylenediamine, melamine, or othernitrogen-containing fluid. The source material may comprise singlecrystal or polycrystalline GaN. The single crystal or polycrystallineGaN for the source material may be grown by any number of methods knownin the art. Other forms of source material may also be used, forexample, amorphous GaN or a GaN precursor such as Ga metal or a Gacompound. It is preferred that the source GaN comprise one or moreparticles that are sufficiently large in size so as not to pass throughthe openings in a baffle, described below, that separates the sourceregion, where the source material is located, from the crystal growthregion, where a nucleation center is located, of a chamber or capsule,as discussed in more detail below.

Nucleation for GaN growth may be induced on the crystal growth portionof the capsule at a nucleation center without a seed crystal, such as aportion of the container wall, or with a non-GaN seed crystal such assapphire, for example. It is preferred, however, that a GaN seed crystalis provided, because the process is easier to control and the quality ofthe grown crystal is higher.

The seed crystal is preferably larger than 1 mm in at least x or ydimension and of high quality, substantially free of tilt boundaries andhaving a dislocation density less than about 10⁸ cm⁻², and preferablybelow about 10⁵ cm⁻². A variety of types of GaN seed crystals may beprovided, including an epitaxial GaN layer on a non-GaN substrate suchas sapphire or SiC, a free-standing GaN film grown by HVPE, sublimation,or metal organic chemical vapor deposition (MOCVD), or a crystal grownin a supercritical fluid in a previous run.

The source material and one or more seeds, if used, are placed in apressure vessel or capsule that is divided into at least two regions bymeans of a porous baffle. An exemplary capsule is described in U.S.patent application Ser. No. 09/683,659 to D'Evelyn et al. filed on Jan.31, 2002, and entitled “High Temperature Pressure Capsule For ProcessingMaterial in Supercritical Fluids”, hereby incorporated by reference inits entirety.

FIG. 1 illustrates an exemplary capsule 100. The capsule 100 includes awall 102, which can be sealed to surround a chamber 104 of the capsule100. The chamber is divided into a first region 108 and a second region106 separated by a porous baffle 110. During crystallization growth thecapsule 100 contains a seed crystal 120 or other nucleation center and asource material 124 separated from each other by the baffle 110. Thesource material 124 and the seed crystal 120 may be positioned in thesecond region 106 and the first region 108, respectively, for example.The capsule 100 also contains a solvent material 130. During the growthprocess, described below, a grown crystal 132 is grown on the seedcrystal 120 and the solvent is in a supercritical state.

The baffle 110 may comprise, for example, a plate with a plurality ofholes in it, or a woven metal cloth. The fractional open area of thebaffle 110 may be between 1% and 50%, and preferably between about 5%and about 40%. Transport of nutrient from the source material 124 to theseed crystal 120 or grown crystal 132 is optimized in the solvent as asupercritical fluid if the colder portion of the capsule 100 is abovethe warmer portion, so that self-convection stirs the fluid. In manysolvents the solubility of GaN increases with temperature, and in thiscase the source material 124 should be placed in the bottom warmerportion of the capsule and the seed crystal 120 in the top colderportion of the capsule.

The seed crystal 120 is preferably hung, for example, by a wire (150)through a hole drilled through the seed, so as to allow crystal growthin all directions with a minimum of interference from wall 102 or othermaterials. The hole may be drilled by a laser, a diamond or abrasivedrill, or an ultrasonic drill, for example. The seed crystal 120 mayalternatively be hung by tying a wire around one end of the seed.

In the case of some solvents, however, the solubility of GaN decreaseswith temperature. In this case the seed crystal 120 should be placed inthe lower warmer portion of the capsule and the source material 124 inthe upper colder portion of the capsule. The source material 124 ispreferably placed in a porous basket 140 displaced from the baffle 110rather than immediately contacting the baffle 110, as the latterarrangement may impede transport of fluid and nutrient through thebaffle 110.

A mineralizer may also be added to the capsule 100, in order to increasethe solubility of GaN in the solvent, either together with the sourcematerial 124 or separately. The mineralizer may comprise at least one of(i) alkali and alkaline-earth nitrides, such as Li₃N, Mg₃N₂, and Ca₃N₂;(ii) amides, such as LiNH₂, NaNH₂, and KNH₂; (iii) urea and relatedcompounds; (iv) ammonium salts, such as NH₄F and NH₄Cl; (v) halide,sulfide, or nitrate salts, such as NaCl, Li₂S, or KNO₃; (vi) azidesalts, such as NaN₃; (vii) other Li salts; (viii) combinations of theabove; and (ix) compounds formed by chemical reaction of at least one ofthe above with Ga and/or GaN.

Optionally, a dopant source is also added to capsule 100, in order toprovide n-type or p-type GaN crystals. Adventitious impurities such asoxygen or carbon will otherwise normally render the crystals n-type.Dopants such as Si (n-type) and Mg or Zn (p-type) may be added asimpurities in the source GaN. Alternatively, the dopants may be added asmetals, salts, or inorganic compounds, such as Si, Si₃N₄, SICl₄, Mg₃N₂,MgF₂, Zn, ZnF₂, or Zn₃N₂.

The capsule 100 is filled with a solvent 130 that will comprise asupercritical fluid under processing conditions, such as, for example,ammonia, hydrazine, methylamine, ethylenediamine, melamine, or othernitrogen-containing fluid. In a preferred embodiment ammonia is employedas the solvent 130. Of the free volume in the capsule, i.e., the volumenot occupied by the source material, seed(s), and baffle), between 25%and 100%, or preferably between 70% and 95%, is filled with solvent 130and the capsule 100 is sealed.

Methods for filling and sealing the capsule are described in U.S.application Ser. No. 09/683,659, filed Jan. 31, 2002 mentioned above,for example. For example, the capsule 100 may be cooled to a temperatureat which the solvent 130 is either a liquid or solid. Once the capsule100 is sufficiently cooled, a solvent source is placed in fluidcommunication with the open chamber of the capsule 100 and solvent isintroduced into the chamber, which is open at this point, by eithercondensation or injection. After a desired amount of solvent 130 isintroduced into the open chamber, the chamber is sealed. The chamber maybe sealed, for example, by pinching off or collapsing a portion of thewall 102 to form a weld.

The sealed capsule 100 is placed in a vessel capable of generatingtemperatures between about 550° C. and about 3000° C., or preferablybetween about 550° C. and about 1200° C. and a pressure between about 5kbar and about 80 kbar, or preferably between about 5 kbar and about 20kbar. An exemplary pressure vessel is described in U.S. application Ser.No. 09/683,658, to D'Evelyn et al. filed on Jan. 31, 2002, and entitled“Improved Pressure Vessel”, hereby incorporated by reference in itsentirety.

FIG. 2 illustrates a pressure vessel 210, with enclosed capsule 100. Thepressure vessel 210 illustrated in FIG. 2 is a hydraulic press with adie. Alternatively, the pressure vessel may comprise a multi-anvil pressor may comprise a die and reinforced end flanges as described in U.S.application Ser. No. 09/683,658 mentioned above.

The pressure vessel 210 contains a pressure medium 214 enclosed bycompression die 204 and top and bottom seals 220 and 222. The pressuremedium may be, for example, NaCl, NaBr or NaF.

The pressure vessel 210 includes a wattage control system 216 forcontrolling the heating of the capsule 100. The wattage control system216 includes a heating element 218 to provide heating to the capsule100, and a controller 222 for controlling the heating element 218. Thewattage control system 216 also preferably includes at least onetemperature sensor 224 proximate to the capsule 100 for generatingtemperature signals associated with the capsule 100.

The pressure vessel 210 is preferably arranged to provide a temperaturedistribution, i.e., the temperature as a function of the position withinthe capsule chamber, within the capsule chamber, including a temperaturegradient within the capsule 100. In one embodiment, the temperaturegradient may be achieved by placing the capsule 100 closer to one end ofthe cell (the region within the pressure vessel 210) than the other.Alternatively, the temperature gradient is produced by providing atleast one heating element 218 having a non-uniform resistivity along itslength. Non-uniform resistivity of the at least one heating element 218may be provided, for example, by providing at least one heating element218 having a non-uniform thickness, by perforating the at least oneheating element 218 at selected points, or by providing at least oneheating element 218 that comprises a laminate of at least two materialsof differing resistivity at selected points along the length of the atleast one heating element 218. In one embodiment, the at least onetemperature sensor 224 comprises at least two independent temperaturesensors provided to measure and control the temperature gradient betweenthe opposite ends 230, 232 of the capsule 100. In one embodiment,closed-loop temperature control is provided for at least two locationswithin the cell. The at least one heating element 218 may also comprisemultiple zones which may be individually powered to achieve the desiredtemperature gradient between two ends of the capsule 100. An exemplaryapparatus and method for providing independent temperature control of atleast two locations within a high pressure cell is described in U.S.patent application “High pressure/high temperature apparatus withimproved temperature control for crystal growth,” to D'Evelyn et al.filed on Dec. 18, 2002, which is incorporated by reference in itsentirety.

The capsule 100 is heated to the growth temperature, preferably betweenabout 550° C. and 1200° C., at an average rate between about 1° C./hrand 1000° C./hr. A temperature gradient may be present in the capsule,due to asymmetric placement of the capsule in the cell, non-symmetricheating, or the like, as described above with respect to the pressurecell 210. This temperature gradient has the effect of creating asupersaturation throughout the heating sequence, which the inventorshave found promotes spontaneous nucleation.

In an embodiment of the present invention the temperature gradient atthe growth temperature is initially held small, less than about 25° C.and preferably less than about 10° C., for a period between about 1minute and 2 hours, in order to allow the system to equilibrate in anequilibrium stage. The temperature gradient as used in this applicationis the difference in the temperature at the ends of the capsule, forexample, where the control thermocouples are located. The temperaturegradient at the position of the seed crystal 120 or nucleation centerwith respect to the temperature at the position of the source material124 is likely to be somewhat smaller.

Optionally, the temperature gradient is set in the equilibrium stage tobe opposite in sign to that where crystal growth occurs on thenucleation center (i.e., so that etching occurs at the nucleation centerand growth occurs on the source material) so as to etch away anyspontaneously-nucleated crystals in the region of the capsule where thenucleation center is provided that may have formed during heating. Inother words, if the crystal growth occurs for a positive temperaturegradient, then the temperature gradient is set to be negative, and viceversa.

After this equilibration period, a growth period may be provided wherethe temperature gradient is increased in magnitude and has a sign suchthat growth occurs at the seed crystal at a greater rate. For examplethe temperature gradient may be increased at a rate between about 0.01°C./hr and 25° C./hr, to a larger value where growth is faster. Duringthe crystal growth the temperature gradient may be held at a magnitudeof between 5° C. and 30° C. and may be adjusted upward or downwardduring growth. Optionally, the temperature gradient may be changed tohave a sign opposite to the sign where growth occurs at the seedcrystal. The sign of the gradient may be reversed one or more additionaltimes in order to alternately etch away any spontaneously-formed nucleiand promote growth on one or more nucleation centers or seed crystals120.

At the conclusion of the growth period the temperature of the capsulemay be ramped down at a rate between about 1° C./hr and 1000° C./hr, andpreferably between about 1° C./hr and 300° C./hr so as to minimizethermal shock to the grown crystal 132. The cell, including the capsuleand pressure medium, is removed from the pressure vessel 210 and thecapsule 100 is removed from the cell.

The solvent 130 may be conveniently removed by chilling the capsule toreduce the vapor pressure of the solvent below 1 bar, puncturing thecapsule, then allowing it to warm so that the solvent evaporates. Thecapsule is cut open and the grown crystal(s) removed. The crystal(s) maybe washed by an appropriate wash, such as, for example, at least one ofwater, alcohol or other organic solvent, and mineral acids to removemineralizer.

The quality of the single crystal may be indicated by characterizationtechniques, such as photoluminescence, which occurs at the band edge atroom temperature for GaN. The crystal may be further processed andsliced into one or more wafers, lapped, polished, and chemicallypolished. This single crystal gallium nitride crystal, and wafers formedtherefrom, are useful as substrates for electronic and optoelectronicdevices.

The crystal may be characterized by standard methods that are known inthe art. For determining the dislocation density, Cathodoluminescence(CL) and etch pit density are convenient. CL imaging provides anon-destructive measure of dislocation density, and requires no samplepreparation. Dislocations are non-radiative recombination centers inGaN, and therefore appear in CL as dark spots. One can simply measurethe concentration of dark spots in CL images to determine thedislocation density.

A second convenient method, which may be more definitive in some cases,is etch pit density. One such etch method, for example, is a vapor-phaseHCl etch, as described by T. Hino et al., Appl. Phys. Lett. 76, 3421(2000) incorporated by reference.

Both of these methods were applied to the Ga face of a sample ofcommercial-grade HVPE GaN dislocation densities (dark-spot densities oretch pit densities) of 1-2×10⁷ cm⁻² were obtained, in excellentagreement with the values reported by the vendor and other researcherson similar material.

The optical absorption and emission properties of the grown GaN can bedetermined by optical absorption and photoluminescence spectroscopies,as are well known in the art. The electrical properties can bedetermined by Van der Pauw Hall effect measurements, by mercury-probeCV, and by hot-probe techniques.

The crystal may be sliced into one or more wafers by methods that arewell known in the art. The GaN crystal or wafer is useful as a substratefor epitaxial Al_(x)In_(y)Ga_(1-x-y)N films where 0≦x≦1, 0≦y≦1 and0≦x+y≦1, light emitting diodes, laser diodes, photodetectors, avalanchephotodiodes, transistors, diodes, and other optoelectronic andelectronic devices. Exemplary methods of forming homoepitaxial lightemitting diodes and laser diodes on a GaN substrate are described, forexample, in U.S. patent application Ser. No. 09/694,690, filed Oct. 23,2000 by D'Evelyn et al., “Homoepitaxial GaN-based light emitting deviceand method for producing,” which is hereby incorporated by reference.Exemplary methods of forming homoepitaxial photodetectors on a GaNsubstrate are described, for example, in U.S. patent application Ser.No. 09/839,941, filed Apr. 20, 2001 by D'Evelyn et al., “HomoepitaxialGaN-based photodetector and method for producing,” which is herebyincorporated by reference. Exemplary methods of forming avalanchephotodiodes on a GaN substrate are described, for example, in U.S.patent application “Avalanche photodiode for use in harsh environments,”U.S. patent application Ser. No. 10/314,986 to Sandvik et al. filed onDec. 10, 2002, which is hereby incorporated by reference.

The above described embodiments provide improved nucleation control byincluding an equilibration period in the temperature program, in whichthe temperature gradient is substantially reduced, or even set to bezero or negative, with respect to the gradient during crystal growth,and by hanging the seed crystal within the growth chamber. The improvedcrystal growth method provides high quality, large area GaN crystals.

A GaN single crystal formed by the above method was characterized usingetch pit density measurements, photoluminescence, and optical absorptiontechniques. The single crystal formed is characterized by a dislocationdensity below 100 cm⁻¹, a photoluminescence spectrum which peaks at aphoton energy of between about 3.38 and about 3.41 eV at a crystaltemperature of 300° K, and has an optical absorption coefficient below 5cm⁻¹ for wavelengths between 700 nm (red) and 465 nm (blue).

EXAMPLES

The following Comparative Examples (Comparative Examples 1-3) areprovided for comparison to the Examples (Examples 1-4). The ComparativeExamples do not constitute prior art to the present invention, but areprovided for comparison purposes.

Comparative Example 1

0.1 g of NH₄F mineralizer was placed in the bottom of a 0.5 inchdiameter silver capsule. A baffle with 5.0% open area was placed in themiddle portion of the capsule, and 0.31 g of polycrystalline GaN sourcematerial was placed in the upper half of the capsule. The capsule wasthen enclosed within a filler/sealing assembly together with a 0.583inch diameter steel ring. The capsule and filler/sealing assembly weretransferred to a gas manifold and filled with 0.99 g of ammonia. Next, aplug was inserted into the open top end of the capsule, such that a coldweld was formed between the silver capsule and silver plug and the steelring surrounded the plug and provided reinforcement. The capsule wasthen removed from the filler/sealing assembly and inserted in a zerostroke high pressure high temperature (HPHT) apparatus. The cell washeated to approximately 700° C. and held at this temperature for 55hours, with a temperature gradient of approximately 85° C. The cell wasthen cooled and removed from the press. Upon opening the capsule afterventing of the ammonia, numerous spontaneously-nucleated crystals wereobserved at the bottom of the capsule. One crystal approximately 0.36 mmin diameter was selected at random and etched in 10% HCl in Ar at 625°C. for 30 min. No etch pits were observed. The area of the exposedc-face was approximately 5.3×10⁻⁴ cm², indicating that the etch pitdensity was less than (1/(5.3×10⁻⁴ cm²)) or 1900 cm⁻². By contrast, theidentical etching treatment was applied to a 200 μm-thick piece of GaNgrown by hydride/halide vapor phase epitaxy (HVPE), and an etch pitdensity of 2×10⁷ cm⁻² was observed on the Ga face.

Comparative Example 2

Three seeds, weighing 3-4 mg each, were placed in the bottom of a 0.5inch diameter silver capsule along with 0.10 g of NH₄F mineralizer. Abaffle with 5.0% open area was placed in the middle portion of thecapsule, and 0.34 g of polycrystalline GaN source material was placed inthe upper half of the capsule. The capsule was then enclosed within afiller/sealing assembly together with a 0.675 inch diameter steel ring.The capsule and filler/sealing assembly were transferred to the gasmanifold and filled with 1.03 g of ammonia. Next, the plug was insertedinto the open top end of the capsule, such that a cold weld was formedbetween the silver capsule and silver plug and the steel ring surroundedthe plug and provided reinforcement. The capsule was then removed fromthe filler/sealing assembly and inserted in a zero stroke HPHTapparatus. The cell was heated at about 15° C./min to approximately 500°C., then at 0.046° C./min to 700° C., and held at the latter temperaturefor 6 hours, with a temperature gradient of approximately 28° C. Thecell was then cooled and removed from the press. Upon opening thecapsule after venting of the ammonia, numerous spontaneously-nucleatedcrystals were observed at the bottom of the capsule and, despite thevery slow heating rate, very little growth on the seeds occurred,relative to growth on spontaneously-nucleated crystals.

Comparative Example 3

A GaN seed, weighing 10.4 mg, was placed in the bottom of a 0.5 inchdiameter silver capsule along with 0.04 g of NH₄F mineralizer. A bafflewith 5.0% open area was placed in the middle portion of the capsule, and0.74 g of polycrystalline GaN source material was placed in the upperhalf of the capsule. The capsule was then enclosed within afiller/sealing assembly together with a 0.675 inch diameter steel ring.The capsule and filler/sealing assembly were transferred to the gasmanifold and filled with 1.14 g of ammonia. Next, the plug was insertedinto the open top end of the capsule, such that a cold weld was formedbetween the silver capsule and silver plug and the steel ring surroundedthe plug and provided reinforcement. The capsule was then removed fromthe filler/sealing assembly and inserted in a zero stroke HPHTapparatus. The cell was heated at about 15° C./min to approximately 500°C., then at 0.05° C./min to 680° C., and held at the latter temperaturefor 53 hours, with a temperature gradient of approximately 70° C. Thecell was then cooled and removed from the press. Upon opening thecapsule after venting of the ammonia, numerous spontaneously-nucleatedcrystals were observed at the bottom of the capsule despite the veryslow heating rate. The seed did grow significantly, to a weight of 41.7mg and a diameter of about 2.5 mm. However, the weight ofspontaneously-nucleated crystals was more than 10×the weight increase ofthe seed.

Example 1

A small hole was drilled by a high-power laser through a GaN seedcrystal weighing 19.7 mg. The seed was hung by a 0.13-mm silver wirefrom a silver baffle with a 35% open area and placed in the lower halfof a 0.5 inch diameter silver capsule along with 0.10 g of NH₄Fmineralizer. 0.74 g of polycrystalline GaN source material was placed inthe upper half of the capsule. The capsule was then enclosed within afiller/sealing assembly together with a 0.583 inch diameter steel ring.The capsule and filler/sealing assembly were transferred to a gasmanifold and filled with 0.99 g of ammonia. Next, the plug was insertedinto the open top end of the capsule, such that a cold weld was formedbetween the silver capsule and silver plug and the steel ring surroundedthe plug and provided reinforcement. The capsule was then removed fromthe filler/sealing assembly and inserted in a zero stroke HPHTapparatus. The cell was heated at a rate of about 11° C./min until thetemperature of the bottom of the capsule was approximately 700° C. andthe temperature of the top half of the capsule was approximately 660°C., as measured by type K thermocouples. The current through the tophalf of the heater was then increased until the temperature gradient ΔTdecreased to zero. After holding at ΔT=0 for 1 hour, the temperature ofthe top half of the capsule was decreased at 5° C./hr until ΔT increasedto approximately 35° C., and the temperatures were held at these valuesfor 78 hr. The cell was then cooled and removed from the press. Uponopening the capsule after venting of the ammonia, the seed weight wasobserved to have increased to 33.4 mg. The crystal was characterized byphotoluminescence, using a 266 nm excitation (frequency-quadrupled YAG).The spectra at several temperatures are shown in FIG. 3. Specificallythe crystal sample was characterized by photoluminescence attemperatures of 5K, 20K, 77K and 300K. At all temperatures in the rangeof 5K-300K, the luminescence peak occurs between 3.38 and 3.45 eV.

Example 2

A GaN seed crystal weighing 12.6 mg, obtained from a previous run, washung through a laser-drilled hole by a 0.13-mm silver wire from a silverbaffle with a 35% open area and placed in the lower half of a 0.5 inchdiameter silver capsule. 0.10 g of NH₄F mineralizer and 1.09 g ofpolycrystalline GaN source material were placed in the upper half of thecapsule. The capsule was then enclosed within a filler/sealing assemblytogether with a 0.583 inch diameter steel ring. The capsule andfiller/sealing assembly were transferred to the gas manifold and filledwith 0.95 g of ammonia. Next, the plug was inserted into the open topend of the capsule, such that a cold weld was formed between the silvercapsule and silver plug and the steel ring surrounded the plug andprovided reinforcement. The capsule was then removed from thefiller/sealing assembly and inserted in a zero stroke HPHT apparatus.The cell was heated at a rate of about 11° C./min until the temperatureof the bottom of the capsule was approximately 700° C. and thetemperature of the top half of the capsule was approximately 640° C., asmeasured by type K thermocouples. The current through the top half ofthe heater was then increased until the temperature gradient ΔTdecreased to zero. After holding at ΔT=0 for 1 hour, the temperature ofthe top half of the capsule was decreased at 5° C./hr until ΔT increasedto approximately 50° C., and the temperatures were held at these valuesfor 98 hr. The cell was then cooled and removed from the press. Uponopening the capsule after venting of the ammonia, the seed had grown toa weight of 24.3 mg. The crystal was then etched in 10% HCl in Ar at625° C. for 30 min. Some etch pits were observed on the c-face above theregion of the seed, with an etch pit density of about 10⁶ cm⁻². However,the areas that grew laterally with respect to the seed were free of etchpits. The area of newly laterally-grown GaN was approximately 3.2×10⁻²cm², indicating that the etch pit density was less than (1/3.2×10⁻² cm²)or 32 cm⁻².

Example 3

Two GaN seeds, weighing 48.4 mg and 36.6 mg and obtained from a previousrun, were hung through laser-drilled holes by a 0.13-mm silver wire froma silver baffle with a 35% open area and placed in the lower half of a0.5 inch diameter silver capsule. 0.10 g of NH₄F mineralizer and 1.03 gof polycrystalline GaN source material were placed in the upper half ofthe capsule. The capsule was then enclosed within a filler/sealingassembly together with a 0.583 inch diameter steel ring. The capsule andfiller/sealing assembly were transferred to the gas manifold and filledwith 1.08 g of ammonia. Next, the plug was inserted into the open topend of the capsule, such that a cold weld was formed between the silvercapsule and silver plug and the steel ring surrounded the plug andprovided reinforcement. The capsule was then removed from thefiller/sealing assembly and inserted in a zero stroke HPHT apparatus.The cell was heated at about 11° C./min until the temperature of thebottom of the capsule was approximately 700° C. and the temperature ofthe top half of the capsule was approximately 642° C., as measured bytype K thermocouples. The current through the top half of the heater wasthen increased until the temperature gradient ΔT decreased to zero.After holding at ΔT=0 for 1 hour, the temperature of the top half of thecapsule was decreased at 5° C./hr until ΔT increased to approximately30° C., and the temperatures were held at these values for 100 hr. Thecell was then cooled and removed from the press. Upon opening thecapsule after venting of the ammonia, the seeds had grown to a weight of219.8 mg. A piece broke off from the smaller of the two crystals and wasselected for analysis. An optical transmission spectrum of the crystalwas measured using a Cary 500i spectrometer. The transmission wasgreater than 60% for wavelengths ranging from red (700 cm⁻¹) to blue(465 cm⁻¹). Based on the index of refraction for GaN [G Yu et al.,Applied Physics Letters 70, 3209 (1997)] and the thickness of thecrystal, 0.206 mm, the optical absorption coefficient was less than 5cm⁻¹ over the same wavelength range. The crystal was determined to haven-type electrical conductivity by means of a hot-point probemeasurement. The crystal was then etched in 10% HCl in Ar at 625° C. for30 min. The entire crystal was free of etch pits. The area of the c-faceof the crystal was approximately 4.4×10⁻² cm², indicating that the etchpit density was less than (1/4.4×10⁻² cm²) or 23 cm⁻².

Example 4

A GaN seed weighing 25.3 mg, obtained from a previous run, was hungthrough a laser-drilled hole by a 0.13-mm silver wire from a silverbaffle with a 35% open area and placed in the lower half of a 0.5 inchdiameter silver capsule. 0.10 g of NH₄F mineralizer and 0.98 g ofpolycrystalline GaN source material were placed in the upper half of thecapsule. The capsule was then enclosed within a filler/sealing assemblytogether with a 0.583 inch diameter steel ring. The capsule andfiller/sealing assembly were transferred to the gas manifold and filledwith 1.07 g of ammonia. Next, the plug was inserted into the open topend of the capsule, such that a cold weld was formed between the silvercapsule and silver plug and the steel ring surrounded the plug andprovided reinforcement. The capsule was then removed from thefiller/sealing assembly and inserted in a zero stroke HPHT apparatus.The cell was heated at about 11° C./min until the temperature of thebottom of the capsule was approximately 700° C. and the temperature ofthe top half of the capsule was approximately 648° C., as measured bytype K thermocouples. The current through the top half of the heater wasthen increased until the temperature gradient ΔT decreased to 3° C.After holding at ΔT=3° C. for 1 hour, the temperature of the top half ofthe capsule was decreased at 5° C./hr until ΔT increased toapproximately 30° C., then decreased further at 2.5° C./hr until ΔTincreased to approximately 60° C. and the temperatures were held atthese values for 20 hr. The cell was then cooled and removed from thepress. Upon opening the capsule after venting of the ammonia, the seedhad grown to a weight of 40.2 mg. The crystal was then etched in 50%HNO₃ for 30 min. A row of etch pits was observed on the c-face above theinterface between the seed and new, laterally-grown material. However,the remaining areas of newly-grown GaN were free of etch pits. The areaof pit-free newly grown GaN was approximately 6.9×10⁻² cm², indicatingthat the etch pit density was less than (1/6.9×10⁻² cm²) or 14 cm-².

The improved methods for forming GaN crystal material described aboveenable the growth of larger high-quality GaN crystals. These improvedGaN crystals enable the fabrication of better-performing electronic andoptoelectronic devices, with improved efficiency, reliability, yields,high power performance, breakdown voltage, and reduced dark current andnoise.

While the invention has been described in detail and with reference tospecific embodiments thereof, it will be apparent to one skilled in theart that various changes and modifications can be made therein withoutdeparting from the spirit and scope of the invention. Thus, the breadthand scope of the present invention should not be limited by any of theabove-described exemplary embodiments, but should be defined only inaccordance with the following claims and their equivalents.

1. A GaN single crystal having a thickness w and dimensions x and ydefining a crystal plane perpendicular to the thickness w, wherein theGaN single crystal is at least about 2.75 millimeters in at least onedimension x or y, with a dislocation density less than about 10⁴ cm⁻²,and substantially free of tilt boundaries, wherein the single crystal isa boule.
 2. The GaN single crystal of claim 1, wherein the GaN singlecrystal is grown from a single seed or nucleus.
 3. The GaN singlecrystal of claim 1, wherein the single crystal is optically transparent,with an optical absorption coefficient below 100 cm⁻¹ at wavelengthsbetween 465 and 700 nm.
 4. The GaN single crystal of claim 3, whereinthe optical absorption coefficient is below 5 cm⁻¹ at wavelengthsbetween 465 and 700 nm.
 5. The GaN single crystal of claim 1, whereinthe single crystal comprises one of n-type and p-type semiconductormaterial.
 6. The GaN single crystal of claim 5, wherein the singlecrystal comprises an n-type semiconductor material and is opticallytransparent, with an optical absorption coefficient below 100 cm⁻¹ atwavelengths between 465 and 700 nm.
 7. The GaN single crystal of claim1, wherein the single crystal has a photoluminescence spectrum peakingat a photon energy of between about 3.38 and about 3.41 eV at a crystaltemperature of 300 K.
 8. The GaN single crystal of claim 1, wherein thedislocation density is less than about 1000 cm⁻².
 9. The GaN singlecrystal of claim 8, wherein the dislocation density is less than about100 cm⁻².
 10. A semiconductor structure comprising a GaN single crystalhaving a thickness w and dimensions x and y defining a crystal planeperpendicular to the thickness w, the GaN single crystal is at leastabout 2.75 millimeters in at least a dimension x or y, has a dislocationdensity less than about 10⁴ cm⁻², and is substantially free of tiltboundaries, wherein the single crystal is a boule.
 11. The semiconductorstructure of claim 10, wherein the GaN single crystal is grown from asingle seed or nucleus.
 12. The semiconductor structure of claim 10,wherein the single crystal is optically transparent, with an opticalabsorption coefficient below 100 cm⁻ at wavelengths between 465 and 700nm.
 13. The semiconductor structure of claim 12, wherein the opticalabsorption coefficient is below 5 cm⁻¹ at wavelengths between 465 and700 nm.
 14. The semiconductor structure of claim 10, wherein the singlecrystal comprises one of n-type and p-type semiconductor material. 15.The semiconductor structure of claim 14, wherein the single crystalcomprises an n-type semiconductor material and is optically transparent,with an optical absorption coefficient below 100 cm⁻¹ at wavelengthsbetween 465 and 700 nm.
 16. The semiconductor structure of claim 10,wherein the single crystal has a photoluminescence spectrum peaking at aphoton energy of between about 3.38 and about 3.41 eV at a crystaltemperature of 300K.
 17. The semiconductor structure of claim 10,wherein the dislocation density is less than about 1000 cm⁻².
 18. Thesemiconductor structure of claim 17, wherein the dislocation density isless than about 100 cm⁻².